With the development of power conversion technologies, high efficiency and high power density has become an important development trend. For its soft switching, high efficiency, high operating frequency, small size and other advantages, resonant converter has been widely used and has gained great attention in the application of the switch mode power supply technology. As a simple circuit topology, series resonant converter is capable of meeting the requirement for high frequency and achieving relatively high conversion efficiency and therefore has been widely applied in the industry.
Taking an LLC (short for Lr, Lm and Cr which represent resonance parameters, i.e., resonant inductance, excitation inductance and resonant capacitance respectively) series resonant converter as an example, when resonant elements on the converter work under a sinusoidal resonance condition, the voltage of a switching tube passes zero naturally to realize the zero-voltage switch-on or switch-off of a primary-side switching tube, resulting in a very small power consumption. Thus, this topology generally adopts a Pulse Frequency Modulation (PFM) control mode.
FIG. 1 shows the main circuit topology of a half-bridge LLC series resonant converter. When the circuit performs PFM control, duty cycles of power tubes Q1 and Q2 are both 0.5, thus, the control is performed through the complementary frequency modulation of fixed dead zones. FIG. 2 shows an equivalent circuit of the resonant network of the main circuit of an LLC series resonant converter, it can be seen from the equivalent circuit diagram that the direct voltage gain of the main circuit of the LLC series resonant converter can be expressed by the following expression:
                    M        =                              2            ⁢            n            ⁢                                                  ⁢                                          V                O                                            V                                  i                  ⁢                                                                          ⁢                  n                                                              =                      1                          2              ⁢              n              ⁢                                                                                          (                                              1                        +                                                                              L                            r                                                                                L                            m                                                                          -                                                                              L                            r                                                                                                                                              L                                m                                                            ⁡                                                              (                                                                                                      f                                    s                                                                    /                                                                      f                                    r                                                                                                  )                                                                                      2                                                                                              )                                        2                                    +                                                                                    Q                        2                                            ⁡                                              (                                                                                                            f                              s                                                                                      f                              r                                                                                -                                                                                    f                              r                                                                                      f                              s                                                                                                      )                                                              2                                                                                                          (        1        )            in which Vo is an output voltage, Vin is an input voltage, fs is a operating frequency (that is, the switch-on frequency of a switching tube), fr is the resonance frequency of a first working region, expressed by
            f      r        =          1              2        ⁢        π        ⁢                                            C              r                        ⁢                          L              r                                            ,Lr is resonant inductance, Lm is excitation inductance, and Q is a quality factor which is expressed as follows:
                    Q        =                                            Z              O                                      R                              a                ⁢                                                                  ⁢                c                                              =                                    1                              R                                                                                          ⁢                                      a                    ⁢                                                                                  ⁢                    c                                                                        ⁢                                                                                L                    r                                                        C                    r                                                              .                                                          (        2        )            
FIG. 3 is a schematic diagram showing the gain characteristic curve of the LLC series resonant converter drawn based on expression (1), and as shown in FIG. 3, the gain characteristic curve of the LLC series resonant converter can be divided into three regions by performance characteristic. The LLC series resonant converter is in a first working region when fs is greater than fr, it is in a second working region when fs is greater than fm but smaller than fr and in a third working region when fs is smaller than fm. In expression (2), Cr is resonant capacitance and fm is the resonance frequency of the second working region and is expressed as follows:
      f    m    =            1              2        ⁢        π        ⁢                                            C              r                        ⁡                          (                              Lr                +                Lm                            )                                            .  
When the operating frequency fs is greater than the resonance frequency fr, the excitation inductor Lm, as a load, takes no part in resonance, then the working mode of the LLC series resonant converter is analogous to that of an ordinary series resonant converter (SRC). When the converter outputs no load or a light load, Rac approaches infinity, Q approximates to 0, thus, the expression (1) can be simplified as follows:
                    M        =                              1                          2              ⁢              n                                ⁢                                    1                              1                +                                                      L                    r                                                        L                    m                                                  -                                                                            L                      r                                                              L                      m                                                        ⁢                                                            (                                                                        f                          r                                                                          f                          s                                                                    )                                        2                                                                        .                                              (        3        )            
It can be seen from the expression (3) that the operating frequency of the LLC series resonant converter rises or the resonance frequency of the LLC series resonant converter falls when the LLC series resonant converter works in a no-load or light-load state, resulting in a reduction in voltage gain; when fs is greater than fr, a small change of the gain will cause a great frequency change, thus, the output voltage of the LLC series resonant converter is hardly stable when the LLC series resonant converter is unloaded or lightly loaded. It can be seen from FIG. 3 that the gain characteristic curve trends towards flat when the LLC series resonant converter works at a low voltage when lightly loaded, to stabilize the voltage, an extremely high operating frequency is required, however, a series of problems will be caused when the operating frequency is extremely high, for instance, the optimization of a magnetic device becomes difficult, the switching loss is increased, and the reliability is lowered; moreover, when the load approximates to empty, the output voltage increases as the frequency or duty cycle of the LLC series resonant converter rises, making it impossible to control a negative feedback loop.
Currently, the following methods are adopted in the industry to overcome the problems above:
Method 1: a small dummy load is added under the no-load or light-load condition to implement regulation of the output voltage;
Method 2: under the no-load or light-load condition, a width modulation (or phase shift) control is performed, that is, the duty cycle (or phase shifting angle) of the switching tube is adjusted;
Method 3: under the no-load or light-load condition, a hybrid control combining width modulation (or phase shift) with frequency and width modulation is performed, that is, the duty cycle (or phase shifting angle) and the operating frequency of the switching tube are adjusted synchronously.
The foregoing three methods, although capable of overcoming the problems above, respectively have the following problems:
Method 1 achieves the voltage stabilization under a no-load or light-load condition but sacrifices the conversion efficiency of the converter when it is unloaded or lightly loaded;
Although method 2 greatly improves stability and implements voltage stabilization under a no-load or light-load condition, due to the nonlinearity of the gain characteristic curve during a width modulation process, the output voltage may decreases as the duty cycle increases, making it difficult to design a loop and hard to guarantee a feedback loop to be invariably stable and not to oscillate in a width modulation range; moreover, when the converter works near a load switching point, the switching between two control polices leads to the instability of the loop and undermines the overall output characteristic of the converter.
On the basis of method 2, method 3 adds frequency and width modulation control to guarantee the linear relationship of an output gain characteristic curve, this method partially eliminates the difficulty in loop design. However, practically, for a converter having a wide output range, it is hard to guarantee the output gain characteristic curve to be linear as the duty cycle varies when an extremely low voltage is output. Therefore, this method also suffers such practical engineering problems that it is difficult to control a loop and optimize a frequency and width modulation curve. Further, when the converter works near a load switching point, the switching between two control polices also leads to the instability of the loop and undermines the overall output characteristic of the converter.
The problems and defects of an unloaded or lightly-loaded converter working at a low voltage are described above by taking a half-bridge LLC series resonant converter as an example, and a full-bridge series resonant converter also confronts the same phenomenon. Theoretically, all series resonant circuits adopting frequency modulation control suffer the problems above.